1. Field of the Invention
The present invention relates to light emitting diodes (LEDs) and, more specifically, to an LED that is capable of functioning as an efficient emitter or detector of light at a common wavelength.
2. Description of the Related Art
LEDs are semiconductor photon sources that can serve as highly efficient electronic-to-photonic transducers. They are typically forward-biased p-n junctions fabricated from a direct-bandgap semiconductor material that emit light via injection electroluminescence. Their small size, high efficiency, high reliability, and compatibility with electronic systems make them very useful for such applications as lamp indicators; display devices; scanning, reading, and printing systems; fiber-optic communication systems; and optical data storage systems.
An LED can emit light efficiently only if the photon energy (E.sub.hv) of the light is substantially less than the bandgap energy (E.sub.g) of the material through which the photons must pass (typically the semiconductor material on each side of the p-n junction). This condition results in low absorption of the emitted photons as they propagate through the semiconductive material. Direct-band-gap LEDs are typically manufactured using standard liquid-phase epitaxy (LPE) techniques in which successive layers of p-doped and n-doped semiconductor material layers are grown on a substrate.
An efficient LED is the graded-bandgap LED described in L. Ralph Dawson, "High Efficiency Graded-Band-Gap Ga.sub.1-x Al.sub.x As Light-Emitting Diodes", Journal of Applied Physics, vol. 48, No. 6 (1977), pp. 2485-2492. This LED utilizes a graded-bandgap structure in which E.sub.g decreases monotonically from a high value on one side of the semiconductive material to a low value on the opposite side. The graded-bandgap structure causes the LED to emit light in the direction of increasing E.sub.g and results in higher emission efficiencies than can be achieved with conventional LEDs. Graded-bandgap LEDs are typically implemented with a single epitaxial layer, rather than multiple successive layers. As a result, they are simpler and less expensive to produce than conventional direct-band-gap LEDs.
An LED can be utilized as a light detector by applying a reverse biased voltage to it. When an LED is reverse biased, photons that are absorbed by the semiconductive LED material generate electron-hole pairs which in turn generate an electric current. The magnitude of the electric current is proportional to the amount of light impinging on the LED (the number of photons) and on the percentage of those photons that are absorbed by the LED (the absorption cross-section). For an LED to be an efficient photon absorber, the bandgap energy of the semiconductive material must be significantly less than the photon energy of the incident light. Otherwise, a majority of the photons will pass through the LED material without being absorbed.
It would be very useful to have an LED that is capable of functioning efficiently as both an emitter and a detector of light at a common wavelength. With such an LED, for example, the user of an optical system that includes both emitters and detectors operating at a common wavelength would only have to stock one type of LED. However, when one compares the bandgap energy requirements of an LED that is used as an emitter with the requirements of one used as a detector, it is apparent that the light emitting and light detecting functions require opposing bandgap energy requirements. Generally, LEDs that are efficient emitters of light at a given wavelength are low efficiency detectors of light of the same wavelength. As a result, a single LED is generally not capable of functioning alternately as an emitter and a detector of light at a common wavelength.
One prior LED, described in U.S. Pat. No. 4,202,000 by Jean-Claude Carballes, exhibits efficient performance as both an emitter and detector of light at a common wavelength by operating in an "avalanche" mode. When a diode is operating in an avalanche mode, a free electron that is generated by photon absorption is accelerated by a strong reverse-biased electric field to an energy greater than E.sub.g. As a result, the accelerated electron itself can generated additional electron-hole pairs by impact ionization. The second electron is also accelerated under the effect of the electric field, and it may also be a source of further impact ionization. The process "avalanches", resulting in a multiplication of the number of electrons generated by the absorbed photon.
Although the avalanche phenomenon utilized by the Carballes LED (when operating as a detector) compensates for its low absorption cross-section, it suffers from several undesirable characteristics. First, diodes that utilize avalanching generally have longer response times than those that do not. This is because additional time is required for the electrons to cause impact ionizations.
Second, the amplification process is inherently random because each detected photon generates a random number of electrons (the number of electrons generated is a function of the number of impact ionizations that take place). As a result, the current generated by the diode has a random component that fluctuates above and below an average value. These random fluctuations in the current are a source of noise.
Third, the avalanche phenomenon can be unstable because it only occurs in the presence of a high reverse-biased electric field (near the breakdown field) in the active region. The strength of the electric field in the active region can be influenced by the amount of doping and the thickness of the active region. Therefore, any localized variations in the thickness or doping can result in variations in the electric field, which in turn can result in localized uncontrolled avalanches.
In addition, the Carballes LED is manufactured by growing three separate semiconductive epitaxial layers on a GaAs substrate. After each epitaxial layer is grown, the liquid composition of the growth solution must be changed by either vapor doping or by bringing the GaAs substrate into contact with a second solution. This multi-step growth technique is more complicated and costly than the single-step growth technique of Dawson, described above.